1. Field of the Invention
The present invention relates to precision measurement systems using optical interferometric techniques and particularly to the use of such techniques to for the purpose of positioning a transducer over the desired track in a disk storage device.
2. Description of the Prior Art
Optical interferometry measurement systems have found widespread application in the laboratory and other situations where the precision required is greater than can be accomplished with other techniques, especially mechanical measurement techniques. The use of interferometry is particularly attractive where the distance to be measured is relatively short, the required precision is compatible wit the wavelength of the source, and the rest of the system is sufficiently expensive to prevent the measurement portion from becoming an undue proportion of the cost. The availability of monochromatic sources such as lasers has made optical measurement a much more practical technique and expanded the applications into surveying, machine tool control, robotics, and similar fields. The development of semiconductor lasers further expanded the field of application by reducing the size, complexity and cost associated with the radiation source. Although there has been a great improvement in the source itself, for example, the use of small, low cost semiconductor lasers the basic interferometer structure has remain unchanged. This structure is necessarily complex and must be made and aligned with great care to obtain the full accuracy possible with such systems. The attendant cost and size have remained as deterrents to those applications where these are primary considerations. Small machine tools, industrial robots, computer disk files and printers are example of applications which have not been fully exploited because of the size and cost of optical measurement systems.
Semiconductor lasers provide a solution to the size and cost problems associated with other lasers, but they introduce a new set of problems which has inhibited their use. For example, the output from semiconductor lasers is highly temperature dependent. Since most applications require a constant power output from the source, some form of power regulation is required. This leads to some type of power measurement and control system. Because this is virtually a universal requirement, most semiconductor lasers include a photosensor, such as a light sensing diode, to sense the power output of the device, positioned on the reverse face of the device. This diode provides an output signal which approximates the power output from the device. This approach, while not as accurate as actually sampling the output beam, has been widely accepted because of the simplicity it provides. Where extreme accuracy is required, the light sensing diode may be positioned to sense the beam emerging from the output face of the laser.
One of the sources of error with the use of diodes mounted on the laser is the power which is fed back into the laser by reflection. To avoid this error, even simple interferometric systems incorporate some form of beam splitter or other optical component to reduce the intensity of the return beam impinging on the laser. This of course increases the cost, not only by the value of the component and supporting structure, but also by the increased difficulty of manufacturing as a result of the additional alignment required.
In addition to sensitivity to reflected power, semiconductor lasers also suffer from a shift in wavelength as the output power is varied. Any change in wavelength also changes the fringe pattern and therefore the output signal of an interferometer even though the path length stays the same. To avoid this problem, designs have relied on some means for stabilizing output power at the desired level and preventing reflected light from reentering the laser.
The cost of gas and semiconductor laser measurement systems has inhibited the expansion of their use into additional fields. For example, in the data processing industry their application has been generally limited to high precision plotting devices and testing systems. While the positioning requirements for disk storage devices could be satisfied with semiconductor laser interferometers, the other optical elements and associated electronic circuits have been much too expensive and take up too much space. As a result, most head positioning servo systems have continued to rely on reference (servo) information recorded on the disk at the time of manufacture. This approach is satisfactory but typically requires the dedication of recording space that could otherwise be used for data. It also complicates the manufacturing process since the servo information must be recorded on the disk after mounting on the spindle.
In the case of lower performance disk files which use flexible disks or diskettes, the trend has been to employ an open loop transducer positioning system using stepper motors. Flexible disk files have generally used a mechanical coupling such as a metal band between the stepper motor and the recording head so that the position of the stepping motor corresponds to the position of the head. The lack of precision in such as system prevents utilization of the full recording potential of the file. The track-to-track spacing required from the magnetic recording standpoint alone is narrower than can be obtained with stepper motor technology. Track density could therefore be increased if the transducer could be positioned more accurately.
The cost and size constraints have also prevented widespread application of interferometric techniques to other disk storage devices. In the case of high performance files, the measurement system must be placed in a sealed enclosure which cannot be opened except in a tightly controlled environment, making service and alignment difficult. Flexible disk files are so inexpensive that they cannot carry the cost burden imposed by such measurement systems. In all situations, the lack of space within the drive assembly is a serious impediment to the incorporation of optical measurement systems.